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tranny problem, looking for advice on a power amp
john11
post Mar 23 2013, 08:47
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Hi and thanks for reading this post.

My amplifier has been playing up on me and i noticed a swollen capacitor which i replaced with a new one of the same voltage and capacitance.

When reassembled the amp would not work at all, after a few hours of experimentation i noticed a row of transistors which are clipped onto the metal chassis of the amp, obviously this was done by the manufacturer to keep the transistors cool and i had to unclip these to get the board out.

When the transistors are clipped back onto the chassis of the amp so in effect they are all sharing one large heatsink the amp does not work, i gave each transistor its own heatsink and now the amp works fine.

Can anyone tell me what is going on.

Many thanks in advance. John.
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john11
post Mar 24 2013, 08:53
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Hi. Thanks for the replies.

The only thing between the transistors and the chassis was a layer of thermal paste. I checked the chassis and it has nothing connected to it, no ground wires, no mains earth wire, no phono ground.

When i reassembled the amp i did not put on a new layer of paste, just thought i would power it up for a few seconds to see if the recap had fixed the problem, the power supply made a high pitched whining noise.

So i applied new paste and the whining noise went away but now there is no sound, perhaps the old paste was having an insulating effect and separating the transistors from each other and the new paste i am using does not do that.

Can you tell me anything about the transistor cases having to be isolated which you mentioned what should be my next move here.

Many thanks. John.
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Glenn Gundlach
post Mar 24 2013, 10:03
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That depends on the case of the transistor. Many (most) power transistors have a metal back side that is connected to the collector terminal meaning there is power or signal on it. This needs to be insulated from the heatsink though there are rare instances where the entire heatsink is 'floating' and the transistors have no insulators to the heatsink. Also, there are some transistor cases that are insulated and require no extra insulators. The only way to be sure is to check service information and transistor data sheets and pay close attention which you apparently did. Is there any chance you put in an electrolytic backwards (reverse polarity) ?

G
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